Transit cars conventionally include multiple train-lined cars supplied by a common power source. Each car includes multiple motors with each motor driving an axle. For AC (alternating current) propulsion applications, these motors are generally induction motors that are controlled by PWM (pulse width modulating) inverters. These variable frequency, variable voltage inverters are fed with DC (direct current) voltage either from a third rail DC source or from a rectified AC source such as a catenary supply. These inverters control the voltage and frequency of the applied voltage on the induction motors to produce the necessary flux and torque.
When partially-enclosed, self-ventilating motors are used in severe weather conditions such as snow storms, moisture accumulates in the motor windings, and freezing and thawing cycles produce leakage currents to ground. These leakage currents become a problem which can be especially significant at the end of a layover period when the cars remain stationary and the warm motors are cooling down. If the snow melts and then re-freezes, re-melting of snow produces leakage currents when the cars start running again. The leakage currents are generally interpreted as ground faults during normal service, and the cars are disabled by a motor controller.